1. Field of the Invention
The present invention relates to memory devices, and more particularly to a method and apparatus for improved isolation between diffusion lines of the memory device.
2. Background Information
Numerous memory devices, including read only memory (ROM), programmable read only memory (PROM), electrically programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM), etc., are well known and commercially available. In these devices, the memory can be arranged in an array comprising parallel, elongated, spaced-apart diffusion regions which function as the source and drain of the array. In this type of layout, the individual memory cells are arranged in rows and columns, with all cells in a column sharing a single drain diffusion and sharing a single source diffusion. Additionally, the source and drain regions typically alternate, such that adjacent columns of cells share each source and drain region. Strips of polysilicon run perpendicular to the diffusion regions to form the control gates.
Although sharing the same drain region with an adjacent column of cells results in a smaller cell size, this scheme complicates the decoder since, to access a cell, in addition to addressing the drain and word line of the cell, it is necessary to address one of the two source regions on each side of the drain. Additionally, a problem which may occur in such cells is parasitic program disturb, wherein an erased cell may get programmed during the programming of another cell on the same word line. This occurs because programming is typically accomplished by applying a voltage of, e.g. 12 V, on the word line, a voltage of, e.g. 6 V, on the drain line, and grounding the source of the cell to be programmed. In this situation, if the source of another cell on the same word line is allowed to float, it may be turned on temporarily as its capacitance is charged, and hence may be programmed. Therefore, to overcome this problem, most devices require complicated disturb prevention circuitry which, for example, holds the source of the cell that is not being programmed at some voltage during programming.
What is needed is a method for forming a non-volatile memory array with improved isolation between diffusion lines. The method should be compatible with conventional processing and allow for forming a variety of memory devices which do not require complicated decoders or disturb prevention circuitry.